activity.rb

require 'rubygems'
require 'sinatra'
require 'dm-core'

DataMapper.setup(:default, ENV['DATABASE_URL'] || 'sqlite3://my.db')

module Activity
  
  class Dogs
    include DataMapper::Resource

    property :id,          Serial
    property :name,        String
    property :description, Text
    property :created_at,  DateTime
  end
  
  class App < Sinatra::Default

    set :run, false
    
    get '/' do
      '<a href="/new">new dog</a><br><ul>' + (Activity::Dogs.all.collect { |d| "<li><a href='/dog/#{d.id}'>#{d.name}</a>" }).join("\n")
    end
    
    get '/dog/:id' do
      dog = Activity::Dogs[params[:id]]
      "<h1>Dog #{dog.id} is #{dog.name}</h1><p>#{dog.description}</p>"
    end
    
    get '/new' do
      "<form method='post'><input type='text' name='name'><br><textarea name='description'>"
    end
    
    post '/new' do
      Activity::Dogs.create :name => params[:name], :description => params[:description]
      redirect '/'
    end
    
  end
end

config.ru

require 'activity.rb'
run Activity::App

get started

0) download this source
1) create a heroku project
2) git push heroku master
3) heroku console
4) DataMapper.auto_migrate!